1. Field of the Invention
The present invention relates to an in-plane switching (IPS) mode liquid crystal display (LCD) device, and more particularly, to an IPS mode transflective LCD device where reflective efficiency and contrast ratio are improved and a method of fabricating the IPS mode transflective LCD device.
2. Discussion of the Related Art
In general, the LCD device uses the optical anisotropy and polarization properties of liquid crystal molecules to produce an image. Since the LCD device is driven by a lower power as compared with the cathode ray tube (CRT) and has advantages of a small size and a thin profile, the LCD device has been widely used as a flat panel display (FPD) device for a monitor or a television. In addition, since the LCD device has a portability due to a light weight, the LCD device has been widely used as a display device for a notebook computer or a personal portable terminal.
The LCD device includes two substrates facing each other and a liquid crystal layer between the two substrates. Two electrodes are formed on inner surfaces of the two substrates and an electric field is generated by a voltage difference between the two electrodes. Liquid crystal molecules of the liquid crystal layer are re-aligned by the electric field and transmittance of the liquid crystal layer is adjusted. As a result, the LCD device displays an image.
Since the LCD device has a non-emissive type, a light source is required to the LCD device. Accordingly, the LCD device includes a liquid crystal panel having two substrates and a liquid crystal layer between the two substrates and a backlight unit under the liquid crystal panel. The light from the backlight unit is supplied to the liquid crystal panel and the liquid crystal layer adjusts the light according to re-alignment of the liquid crystal molecules, thereby the image displayed.
Since a transmissive LCD device, where a light from the backlight unit passes through the liquid crystal panel, uses an artificial light source such as the backlight unit, the transmissive LCD device displays a bright image under a dark circumstance. However, since the backlight unit consumes a power, the transmissive LCD device has a disadvantage of a relatively large power consumption.
To improve the disadvantage of the transmissive LCD device, a reflective LCD device using an ambient light has been suggested. Since the reflective LCD device uses an ambient natural light or an ambient artificial light, the reflective LCD device has a relatively lower power consumption as compared with the transmissive LCD device. As a result, the reflective LCD device is widely used as a display device for a portable terminal such as a personal digital assistant (PDA). Although the reflective LCD device without an additional light source has an advantage in power consumption, the reflective LCD device cannot be used with a week ambient light or without an ambient light.
Recently, a transflective LCD device having advantages of the transmissive LCD device and the reflective LCD device has been suggested. The transflective LCD device may have one of an electrically controlled birefringence (ECB) mode and a vertical alignment (VA) mode. However, the ECB mode transflective LCD device has a disadvantage of a narrow viewing angle, and the VA mode transflective LCD device has a disadvantage of a high production cost for a plurality of compensation films.
The LCD device is not applied only to a personal display device such as a notebook computer but also to a used for a mass media display device such as a television. As a result, a plurality of users watch the LCD device along various angles and the viewing angle of the LCD device has been the subject of recent research. Accordingly, to improve the disadvantages of a narrow viewing angle of the ECB mode transflective LCD device and the VA mode transflective LCD device, an in-plane switching (IPS) mode transflective LCD device where a pixel electrode and a common electrode are formed on the same substrate and the liquid crystal molecules are re-aligned by a horizontal electric field has been suggested.
FIG. 1 is a cross-sectional view showing an in-plane switching mode transflective liquid crystal display device according to the related art. In FIG. 1, an in-plane switching (IPS) mode transflective liquid crystal display (LCD) device 1 includes first and second substrates 2 and 83 and a liquid crystal layer 90 between the first and second substrates 2 and 83. The first and second substrates 2 and 83 face and are spaced apart from each other. Although not shown in FIG. 1, a gate line and a data line are formed on an inner surface of the first substrate 2. The gate line crosses the data line to define a pixel region P having a transmissive area TA and a reflective area RA. In addition, although not shown in FIG. 1, a common line parallel to the gate line crosses the pixel region P and a switching thin film transistor (TFT) is disposed in the pixel region P. A reflecting layer 50 of a metallic material having a relatively high reflectance is formed in the reflective area RA, and a pixel electrode 70 is formed over the reflecting layer 50. The pixel electrode 70 is formed in the reflective area RA and the transmissive area TA. An insulating layer 72 is formed on the pixel electrode 70 and a common electrode 80 connected to the common line is formed on the insulating layer 72. The common electrode 80 includes a plurality of first openings op1 and a plurality of second openings op2 each having a bar shape. The plurality of first openings op1 and the plurality of second openings op2 are disposed in the transmissive and reflective areas TA and RA, respectively.
A black matrix (not shown) is formed on an inner surface of the second substrate 83 and a color filter layer 86 is formed on the black matrix. The black matrix corresponds to a border between the adjacent pixel regions P and the color filter layer 86 corresponds to the pixel region P. In addition, an overcoat layer 88 is formed on the color filter layer 86. First and second polarizing plates 93 and 95 are formed on outer surfaces of the first and second substrates 2 and 83, respectively.
The IPS mode transflective LCD device 1 has first and second cell gaps d1 and d2 in the transmissive and reflective areas TA and RA, respectively. The first cell gap d1 which corresponds to a thickness of the liquid crystal layer 90 in the transmissive area TA is twice of the second cell gap d2 which corresponds to a thickness of the liquid crystal layer 90 in the reflective area RA so that retardation of light passing through the liquid crystal layer 90 in the transmissive area TA can be the same as retardation of light passing through the liquid crystal layer 90 in the reflective area RA. The light from a backlight unit (not shown) under the first substrate 2 passes through the liquid crystal layer 90 in the transmissive area TA, while the ambient light passes through the liquid crystal layer 90, the ambient light is reflected at the reflecting layer 50 and the reflected ambient light passes through the liquid crystal layer 90 again in the reflective area RA. As a result, the light passes through the liquid crystal layer 90 once in the transmissive area TA, while the light passes through the liquid crystal layer 90 twice in the reflective area RA.
Since the retardation of light is proportional to a distance of light path, the retardation of light in the transmissive area TA is different from the retardation of light in the reflective area RA when the first cell gap d1 is the same as the second cell gap d2. To make the retardations in the transmissive and reflective areas TA and RA the same as each other, the liquid crystal layer 90 is formed such that the first cell gap d1 is twice the second cell gap d2 (d1=2×d2). For example, the liquid crystal layer 90 in the transmissive area TA may be formed as a λ/2 cell where the phase of light changes by π, while the liquid crystal layer 90 in the reflective area RA may be formed as a λ/4 cell where the phase of light changes by π/2.
In the transmissive area TA of the IPS mode transflective LCD device 1, a black image is obtained when a director, which is a longer axis of a liquid crystal molecule, of the liquid crystal layer 90 of the λ/2 cell is parallel to a polarization axis of the second polarizing plate 95. In the reflective area RA of the IPS mode transflective LCD device 1, the black image is obtained when the director of the liquid crystal layer 90 of the λ/4 cell has an angle of 45° with respect to the polarization axis of the second polarizing plate 95. Since the orientation directions for each of the first and second substrates 2 and 83 in the transmissive and reflective areas TA and RA are different from each other, the split orientation for the transmissive and reflective areas TA and RA may be performed only through an ultraviolet (UV) orientation method.
However, the IPS mode transflective LCD device 1 has disadvantages of a low reflection efficiency and an increase of a black image brightness in the reflective area RA where the liquid crystal layer 90 has the lower thickness.
FIG. 2 is a graph showing a relation between a reflectance and a voltage difference according to a cell gap in an in-plane switching mode transflective liquid crystal display device according to the related art, FIG. 3 is a graph showing a relation between a reflectance and a voltage difference according to a wavelength in an in-plane switching mode transflective liquid crystal display device according to the related art, and FIG. 4 is a graph showing a relation between a reflectance of a black image and a wavelength in an in-plane switching mode transflective liquid crystal display device according to the related art.
In FIG. 2, as the cell gap decreases, the reflectance in the reflective area decreases when the voltage difference between the pixel electrode and the common electrode is within a range of about 2V to about 7V. For example, when the voltage difference of about 5V is applied to the liquid crystal layer in the reflective area, the reflectance through the liquid crystal layer having the cell gap of about 1.8 μm is about 0.74 and the reflectance through the liquid crystal layer having the cell gap of about 4.3 μm is about 0.85. As a result, the reflectance through the liquid crystal layer having the cell gap of about 1.8 μm is reduced by about 15% of the reflectance through the liquid crystal layer having the cell gap of about 4.3 μm.
Moreover, the brightness of a white image decreases as the cell gap decreases. The white image may be obtained in the reflective area when the voltage difference between the pixel electrode and the common electrode is within a range of about 6V to about 7V. As the cell gap decreases, the uniformity of the directors according to positions is deteriorated. In addition, when the liquid crystal layer is driven by the voltage difference, as the director is re-aligned to be more parallel to the polarization axis of the second polarizing plate, the change in brightness of the white image due to the change of the director increases.
In FIG. 3, the brightness of each of the black images for wavelengths of about 650 nm and about 450 nm is greater than the brightness of the black image for a wavelength of about 550 nm. The black images for the wavelengths of about 650 nm, about 550 nm and about 450 nm may be obtained in the reflective area when the voltage difference between the pixel electrode and the common electrode is about 0V. For example, when the voltage difference of about 0V is applied to the liquid crystal layer in the reflective area, the reflectance for the green-colored light having the wavelength of about 550 nm may be about 0 and the black image for the green-colored light may have a relatively low brightness. However, when the voltage difference of about 0V is applied to the liquid crystal layer in the reflective area, the reflectance of the blue-colored light having the wavelength of about 450 nm may be about 0.23 and the black image for the blue-colored light may have a relatively high brightness.
In FIG. 4, when the voltage difference of about 0V is applied to the liquid crystal layer, the reflectance in the reflective area has a minimum value at the wavelength of about 550 nm and the reflectance in the reflective area increases as the wavelength increases or decreases. For example, the reflectance in the reflective area may be about 0 at the wavelength of about 550 nm, and the reflectance in the reflective area may be greater than about 0 at the other wavelengths. The increase in the brightness of the black image may cause deterioration of contrast ratio of the IPS mode transflective LCD device.